An Organic Light Emitting Display Diode (OLED), as a current-type light emitting device, has been applied to displays with high performance more widely. With an increasing in size of the display, the traditional passive matrix OLED requires shorter drive time for single pixel, and thus an instantaneous current has to be increased, which increases power consumption. Further, applying a large current would cause a voltage drop across ITO line too large and an operation voltage of the OLED too high, and in turn the efficiency of the OLED would decrease. Application of an Active Matrix OLED (AMOLED) device may settle such problem well, since it inputs OLED current by scanning line-by-line through switch transistors.
In designs for backboard of the AMOLED, a main problem to be settled is non-uniformity in brightness among pixels.
Firstly, most of the AMOLED constructs a pixel circuit by utilizing Low Temperature polycrystalline silicon Thin Film Transistor (LTPS TFT) so as to provide corresponding currents to the OLED devices. As compared with the general amorphous-Si TFT, the LTPS TFT has a higher mobility and a more steady character, and is more suitable for being applied in the AMOLED displays. However, the LTPS TFT formed on a glass substrate with a large area often has non-uniformity on electrical parameters such as threshold voltage, mobility, etc. due to a limitation in the crystallization process, and such non-uniformity will lead to a current difference and brightness difference of the OLED display devices which may be perceptible to human eyes, that is, a mura phenomenon occurs.
Secondly, in an application of displays with large size, power lines on the backboard have certain resistance and the driving currents in all of the pixels are provided by the ARVDD, therefore a voltage of power supply in areas near a power supplying position of the ARVDD is higher than that in areas far away from the power supplying position in the backboard. This phenomenon is called as resistance voltage drop (IR Drop). Because the voltage of the ARVDD is relevant to the current, the IR Drop also causes current differences in different areas, and in turn the mura would occur as display.
Thirdly, uneven thickness in the film, when the OLED device is evaporated, also may cause the non-uniformity in the electrical performances. Further, after operating for a long time, a degradation of its internal electrical performances may result in an increased threshold voltage, such that the efficiency of light emitting is low and brightness drops. As shown in FIG. 6(a), the brightness of the OLED device decreases, and its threshold voltage increases gradually, as the usage time increases.
How to compensate the degradation of the OLED device has been an important issue recently, because the degradation of the OLED may cause an occurrence of Image Sticking in areas displaying unchanged pictures for a long time, which affects the display effect.
As shown in FIGS. 6(b), 6(c), the increasing of the threshold voltage of OLED basically has a linear relationship with the brightness loss, and a relationship between the current of OLED and the brightness is also linear. Therefore, when the degradation of the OLED is compensated, we can increase the driving current linearly as the threshold voltage of OLED increases so as to compensate the brightness loss.
The AMOLED may be divided into three classes based on the driving mode: a digital type, a current type and a voltage type. The driving method of digital type realizes grayscale levels by using TFTs as switches to control a driving time without compensating the non-uniformity, but its operation frequency would increase doubly with an increasing of the display size, which results in a large amount of power consumption and would reach the physical limit of design in a certain range, therefore it is not suitable for applications with large display size. The driving method of current type realizes grayscale levels by providing different currents to the drive transistor directly, and it may compensate the non-uniformity of the TFTs and the IR drop well, however, a overlong written time would occur when a small current charges a large parasitic capacitance on the data line, and such problem is specially serious and difficult to be overcome in the large size display. The driving method of voltage type is similar to the traditional driving method for AMLCD and provides a voltage signal indicating grayscale level by a driving IC, and the voltage signal would be converted into a current signal of the drive transistor inside the pixel circuit, so that the OLED is driven to realize grayscale presenting the brightness. Therefore, the driving method of voltage type is used widely in the industry for its rapid driving speed and simply implementation, and is suitable to drive a large size panel, but the non-uniformity of TFTs and IR drop have to be compensated by other TFTs and capacitors designed additionally.
FIG. 7 is a traditional pixel circuit structure of a voltage driving type, comprising 2 TFTs and 1 capacitor (2T1C). A switching transistor T2 transfers the voltage on the data line to the gate of the driving transistor T1, and the driving transistor T1 converts the data voltage to a corresponding current for supplying for the OLED device. In a normal operation, the driving transistor operates in a saturation area and provides a constant current during a period for scanning one line. As shown in following equation (1), the driving current is expressed as:
                              I          OLED                =                              1            2                    ⁢                                    μ              P                        ·            Cox            ·                          W              L                        ·                                          (                                  Vdata                  -                  ARVDD                  -                                      V                    TH                                                  )                            2                                                          (        1        )            
Wherein μP denotes carrier mobility, COX denotes a gate oxide layer capacitance, W/L denotes a ratio of width to length of the transistor, Vdata denotes a data voltage, ARVDD denotes a backboard power supply of the AMOLED shared by all pixel units, and Vth denotes a threshold voltage of the transistor. It can be seen from the above equation, variation occurs in the current if the Vth among different pixel units are different. Further, with the degradation of the OLED device, the brightness of the OLED would decrease even if a constant current is provided.
Document [1] discloses a pixel structure which is capable of compensating the non-uniformity of Vth and IR drop, and the control timing thereof, as shown in FIG. 8. The structure in FIG. 8 may compensate effects due to the non-uniformity of Vth, IR drop and the degradation of OLED, but it is not suitable for the application with a large size panel since it is adopted in a driving method of current type.
It can be seen that, no effectual means for settling the previously-described problems, that is, how to compensate a luminance non-uniformity caused by the degradation of the OLED device, the non-uniformity of the threshold voltage in the TFTs and the voltage drop of the backboard power supply (IR drop), are not proposed in the prior art.
Reference Document
[1] “Current programming pixel circuit and data-driver design for active-matrix organic light-emitting diodes”, Journal of the Society for Information Display 12 (2004) 227